Remove stems and black gills from underside of mushroom. Toss gills and chop stems. Place mushrooms on parchment paper on a large sheet pan. Tops down.

Roughly chop onion and garlic. Place into the bowl of a mini food processor, along with mushroom stems and chop until well minced. Set aside.

Remove sausages from casings and brown in olive oil in a large skillet, adding kosher salt, pepper and red pepper flakes. Cook until sausage is no longer pink in the middle, 5-7 minutes. Add vegetable mixture to skillet, along with 1/2 teaspoon kosher salt and 1/2 teaspoon pepper. Continue to cook for 3-5 minutes.